Method for coating a cylinder sleeve

ABSTRACT

The invention relates to a method for coating a cylinder sleeve consisting of an iron-based material. A first layer and a second layer made of a zinc alloy are injected onto the outer surface using a thermal injection method. A good metallic connection between the cylinder sleeve and the cylinder crankcase, wherein the cylinder sleeve is poured, is produced by using alloyed or unalloyed copper for the first layer and by using a zinc-aluminum-alloy for the second layer.

The invention relates to a method for coating a cylinder sleeve, according to the preamble of claim 1.

Cylinder crankcases are usually cast from light metal, particularly aluminum, whereby there is the need, due to the poor tribological properties of the light metals, to cast cylinder sleeves of an iron-based material, for example gray cast iron, into the cylinder crankcase, as well. In this connection, problems occur with anchoring the cylinder sleeves in the cylinder crankcase in sufficiently firm manner, and with guaranteeing a sufficient heat transfer between the cylinder sleeves and the cylinder crankcase. These problems can be solved in that the outer surfaces of the cylinder sleeves are given a rough-cast structure with undercuts. This brings with it the result that the crosspieces between the cylinder sleeves cast into the cylinder crankcase are very broad, and that therefore the space requirement of the cylinder sleeves is very great.

Within the scope of the trend in engine development, of reducing the size of the engines while maintaining performance, there is the need to reduce the distances between the individual cylinder sleeves, and, at the same time, to improve the heat removal from the combustion chamber to the cooling chambers of the cylinder crankcase, by way of the cylinder sleeve. These problems can be solved in that, as an alternative to a rough-cast sleeve, cylinder sleeves made of gray cast iron having a smooth or moderately rough outer surface and having a coating are used, which coating assures bonding of the cylinder sleeve to the surrounding cast material of the cylinder crankcase.

A cylinder sleeve of this type, made of gray cast iron, is described in the patent DE 197 29 017 C2. This sleeve has a cover layer of an AlSi alloy on its outer surface, applied using the flame-spraying method or the arc-spraying method, which alloy contains less than 15% silicon. An oxidation protection layer is applied to this cover layer, consisting of a zinc alloy, whose task it is to prevent oxidation of the AlSi layer, thereby preventing metallic bonding of the cover layer to the surrounding cast material of the cylinder crankcase.

It is a disadvantage, in this connection, that the AlSi alloy already oxidizes when the cover layer is sprayed on. The oxide skin formed in this connection adheres very firmly to the AlSi layer. Furthermore, its melting temperature is higher than the temperatures that can be reached during surrounding casting. It is true that the oxide skin can be removed with a lot of effort, but it quickly forms again after having been removed, so that even an additionally applied protective layer of zinc or of a zinc alloy cannot assure a metallic bond between the AlSi layer and the surrounding cast material.

Furthermore, the expansion coefficient of the AlSi cover layer is approximately 1.7 times as great as the heat expansion coefficient of gray cast iron, so that tensions in the known layer system occur in the case of temperature changes, which tensions impair the bond between the cylinder sleeve and the cylinder crankcase.

It is the task of the invention to avoid these disadvantages of the state of the art, i.e. to improve the metallic bond of the cylinder sleeve with the surrounding cast material of the cylinder crankcase, and therefore also the heat transfer between the cylinder sleeve and the cylinder crankcase. This task is accomplished with the characteristics standing in the characterizing part of the main claim.

In this connection, the advantages are obtained that the gradation of the thermal expansion coefficients, in the layer structure according to the invention, between the gray cast iron sleeve, the layer system, and the surrounding casting material of the cylinder crankcase, significantly reduces the thermally caused tensions in the layer structure according to the invention. Furthermore, the gradation of the melting temperatures, proceeding from the cylinder sleeve, by way of the layer system according to the invention, all the way to the surrounding cast material of the cylinder crankcase, brings about partial solution, i.e. diffusion-related partial alloying of the outer layer with the surrounding cast material, bringing about a stable metal bond between the cylinder sleeve and the surrounding cast material of the cylinder crankcase. Finally, the coating consisting of the alloys according to the invention has the advantage that its alloy components participate in precipitation hardening in the bonding zone.

Practical embodiments of the invention are the object of the dependent claims.

The method according to the invention, for coating a cylinder sleeve to be cast into a cylinder crankcase, will be explained in greater detail below.

A cylinder sleeve that consists of an iron-based material, which can be alloyed or unalloyed, is used for this purpose. Preferably, the cylinder sleeve consists of gray cast iron, which can contain either lamellar graphite, vermicular graphite, or spherical graphite. In this connection, the gray cast iron can have a ferrite/perlite, perlite, bainite, or austenite basic structure. The outer surface of the cylinder sleeve can be configured to be smooth. However, it can also have any other surface quality, all the way to a flat rough-cast surface. Furthermore, the cylinder sleeve can have an outer surface that has been machined by means of cutting.

Any conventional casting methods, such as the die-casting method, the pressure casting method, the gravity casting method, or the low-pressure casting method, can be used for casting the cylinder sleeve into the cylinder crankcase. The cylinder crankcase consists of one of the usual light metal casting material, whereby casting materials both on an aluminum basis and on a magnesium basis can be used.

In order to assure the metallic bond of the cylinder sleeve to the surrounding casting material of the cylinder crankcase when the cylinder sleeve is cast into the cylinder crankcase, the outer surface of the cylinder sleeve is coated by means of thermal spraying. As preparation for this, it is necessary to clean the outer surface of dirt and oxides, and subsequently to roughen it up. Suitable methods for this are brushing and/or sandblasting. Sandblasting with coarse corundum, i.e. with crystallized Al₂O₃, is particularly suitable for this.

Immediately subsequent to this, a first layer is then applied to the outer surface of the cylinder sleeve, by means of thermal spraying. This first layer consists either of 99.9% copper, a CuAl8 alloy, a CuAl8Ni2 alloy, a CuP8 alloy, a CuSi3 alloy, or a CuZn37 alloy (brass). In this connection, a layer low in pores and oxides, having a thickness between 60 μm and 130 μm, is aimed at.

In order to make the gradation of the melting temperatures of the layers applied to the cylinder sleeve finer, it is advantageous to apply an additional layer of one of the aforementioned copper alloys to a first layer consisting of pure copper, the melting temperature of which additional layer is lower than that of copper, but whose melting temperature is higher than that of the material of which the outer coating, referred to as the second layer in the following, consists.

Since wires of the stated alloys is commercially available, the wire flame-spraying method is preferably used as a thermal spraying method, whereby the additional spray material in wire form is melted in the center of an acetylene-oxygen flame, and sprayed onto the surface of the cylinder sleeve using an atomizer gas, such as compressed air or nitrogen, for example.

The electric arc wire spraying method is also suitable, whereby two spray additives in wire form are melted in an electric arc and centrifuged onto the outer surface of the cylinder sleeve by means of an atomizer gas. In this connection, there is the possibility of melting two wires that differ in their composition with one another, whereby the composition of the layer produced in this manner can be varied within broad ranges. If, for example, copper wire and zinc wire are used, there is the possibility of applying a CuZn alloy having up to 45% zinc onto the outer surface of the cylinder sleeve. When nitrogen or argon is used as the atomizer gas, oxidation of the materials is prevented, to a great extent.

One possibility for further reducing oxidation of the spray material and the oxide content of the sprayed-on layer consists in using the cold gas spraying method, whereby non-melted powder particles, heated only to a few hundred degrees, are accelerated to a velocity between 300 m/sec and 1200 m/sec, and sprayed onto the outer surface of the cylinder sleeve. The temperature at the contact surface increases on the basis of the impact of the powder particles, and results in micro-welding of the powder particles to the outer surface of the cylinder sleeve.

The high-velocity flame spraying method (HVOF spraying method) can also be used, whereby continuous gas combustion takes place, at high pressures, within a combustion chamber in the central axis of which the spraying additive is supplied in powder form. The high pressure of the fuel gas/oxygen mixture produced in the combustion chamber produces a high particle velocity, which leads to very dense spray layers having good adhesion properties.

The functions of the first layer consist in assuring good adhesion of the first layer to the gray cast iron of the cylinder sleeve, creating good bonding prerequisites for a second layer, and implementing a gradation of the melting temperatures, i.e. a step-by-step transition of the melting temperatures of the gray cast iron of the cylinder sleeve, by way of the first layer and the second layer, all the way to the surrounding cast metal of the cylinder crankshaft. Furthermore, in this way, a gradation of the heat expansion coefficients is brought about, proceeding from the cylinder sleeve, by way of the first and the second layer, all the way to the light metal of the cylinder crankcase.

In order to avoid oxidation of the first layer, the second layer is applied to the first layer immediately after application of the first layer, using one of the aforementioned thermal spraying methods. A Zn85Al15 alloy having 85% zinc and 15% aluminum is preferably used for this purpose. In the case of this alloy, however, the aluminum content can also vary between 3% and 20%. A layer low in pores and oxides, having a thickness between 60 μm and 130 μm, is aimed at.

The function of the second layer consists in adhering well to the first layer. Furthermore, the AlZn alloy, having 15 wt.-% aluminum, has a melting point of 450° C., bringing about the result that the second layer is slightly melted when the cylinder crankcase is cast, by its surrounding cast material, thereby assuring the metallic bond between the cylinder sleeve and the surrounding cast material of the cylinder crankcase.

In this connection, the AlZn alloy does form a very thin oxide layer, but this does not hinder bonding of cylinder sleeve/crankcase. Nevertheless, it is advantageous to alloy a few wt.-% of copper into the AlZn alloy, because in this way, formation of the oxide layer is completely prevented, and this brings about a further improvement of the bond between cylinder sleeve and crankcase. 

1. Method for coating a cylinder sleeve made of an iron-based material, whereby a first layer and a second layer of a zinc alloy are sprayed onto the outer surface of the cylinder sleeve, using a thermal spraying method, comprising the following method steps, copper or a copper-based alloy is sprayed onto the outer surface of the cylinder sleeve as a first layer, and a zinc-aluminum alloy is sprayed onto the first layer as a second layer.
 2. Method according to claim 1, wherein 99.9% copper is sprayed on as the first layer.
 3. Method according to claim 1, wherein a CuAl8 alloy is sprayed on as the first layer.
 4. Method according to claim 1, wherein a CuAl8Ni2 alloy is sprayed on as the first layer.
 5. Method according to claim 1, wherein a CuP8 alloy is sprayed on as the first layer.
 6. Method according to claim 1, wherein a CuSi3 alloy is sprayed on as the first layer.
 7. Method according to claim 1, wherein a CuZn alloy having up to 45 wt.-% zinc is sprayed on as the first layer.
 8. Method according to claim 7, wherein a CuZn37 alloy is sprayed on as the first layer.
 9. Method according to claim 1, wherein the first layer has a thickness between 60 μm and 130 μm.
 10. Method according to claim 1, wherein a ZnAl alloy having 3 to 20 wt.-% aluminum is sprayed on as the 2^(nd) layer.
 11. Method according to claim 10, wherein a Zn85Al15 alloy is sprayed on as the 2^(nd) layer.
 12. Method according to claim 10, wherein copper is alloyed into the 2^(nd) layer.
 13. Method according to claim 1, wherein the cold gas spraying method is used for spraying on the 1^(st) and/or the 2^(nd) layer.
 14. Use of copper or of a copper-based alloy as a first layer to be applied to a cylinder sleeve made of gray cast iron, and use of a zinc-aluminum alloy as the second layer to be applied to the first layer. 